Quadraphonic recording system

ABSTRACT

Apparatus for recording four separate channels of sound information on the two tracks of a stereophonic disc record for subsequent reproduction on four loudspeakers to give the listener the illusion of sound coming from four separate sources. Two of the four signals are applied to respective ones of the two tracks, as in stereophonic recording, with the third and fourth channels superimposed thereon by applying equal in-phase portions of the third signal to both of the tracks and by applying equal portions of the fourth signal to both of the tracks with a differential phase angle between the two portions which remains substantially constant throughout the audio frequency range of interest. The fourth signal applied in this way causes the stylus of a stereophonic cutter to describe an ellipse having major vertical and horizonal axes disposed normal and parallel, respectively, to the surface of the record.

United States Patent 1191 Bauer et al.

1 1 Feb. 26, 1974 1/1972 Scheiber 179/1004 ST 1 QUADRAPHONIC RECORDING3,632,886

SYSTEM [75] Inventors: Benjamin B. Bauer, Stamford; Primary Examiner Raymcnd Cardin),

Daniel w Gravereaux, Wilton, both Attorney, Agent, or Fzrm-Spencer E.Olson 1 of Conn.

[73] Assignee: Columbia Broadcasting Systems, 1 ABSTRACT New YorkApparatus for recording four separate channels of 22 l J 13 1972 soundinformation on the two tracks of a stereophonic disc record forsubsequent reproduction on four loud- [21 App]. No.: 271,470 speakers togive the listener the illusion of sound com- Rdated US. Application Dataing from four separate sources. Two of the four signals [62] Diviqion ofSer NO 96 June 8 1970 PM No are applied to respective ones of the twotracks, as in 3 .7O 8.63L I I l y l stereophonic recording, with thethird and fourth channels superimposed thereon by applying equal in- 521U.S. c1 ..179/100.4 ST, 179/1 GQ, P a portions of the third signal toboth of the tracks 179/1001 TD 004 C p and by applying equal portions ofthe fourth signal to [51] Int Cl G'llb 3/74 both of the tracks with adifferential phase angle be- [58] Field or sarill fl il fffI111791100.4ST, 100.4 C, 199Eh9111929 1 which remains substantially 179/100.l TD, 1G, 1 6Q 15 BT Constant throughout the audio frequency range of interest.The fourth signal applied in this way causes the stylus of astereophonic cutter to describe an ellipse [56] References Cited havingmajor vertical and horizonal axes disposed nor- UNITED STATES PATENTSmal and parallel, respectively, to the surface of the re 3,646,5742/1972 Holzer 179/1 (10 cord v l 2 Claims, 9 Drawing Figures 0 .707--1SH/FT 0 ENCODER FOR MULTl-CHANNEL SOUND SYSTEM This is a division ofapplication Ser. No. 44,196 filed .lune 8, 1970, now US. Pat. No,3,708,631.

BACKGROUND OF THE INVENTION There is an increasing interest inmultiplechannel recording and reproduction because of the variety ofsounds and music forms that can be achieved thereby due to the wellknown phenomenon that the quality of music reproduction is enhanced whenthe number of reproduction channels increases. In the early days of thephonograph, only single channel or monaural recording was used, and asearly as 40 to 50 years ago, investigators realized the value ofrecording and transmitting two separate channels of information, whichin modern parlance is known as binaural or stereophonic sound. However,even two channels of information are not considered sufficient for fullillusion of reality. For

example, when a listener is placed in front of a symphony orchestra hehears sounds arriving from many different directions and from a varietyof instruments, as well as reflections from the walls and ceiling, whichgives him an accustomed illusion of space perspective, However, whenreproduction is accomplished by utiliz' ing only two channels it isdifficult, if not impossible, to produce true reality with respect tospatial perspective. Early experiments have demonstrated that a minimumof three independent channels are needed to convey a satisfactoryillusion of reality in the reproduction of orchestral music.

The modern stereophonic phonograph is capable of recording, or encoding,modulation along two separate channels, which geometrically are at 90 toeach other and at 45 to the disc surface. It is usual practice toinclude a third channel by matrixing or adding it as a phantom channelto the other two, which causes it to be recorded as lateral modulationparallel to the record surface. Oftentimes, to obtain special effects,some of the channels are applied to the tracks in phase opposition, in amanner exemplified by test records Models STR 110, 111 and 120 producedand distributed by CBS Laboratories, a Division of the Assignee of thisinvention. Upon reproduction, the third (or central) channel appears onthe two loudspeakers of the stereophonic phonograph, and an observerplaced centrally between the loudspeakers perceives the illusion of thethird channel being located between the other two The fourth, orvertical, channel when reproduced on a conventional two-loudspeakerstereophonic phonograph gives the illusion of spread sound. Althoughthere have been attempts to reproduce the third or center channel on aseparate loudspeaker, the results have not been entirely satisfactory,and most stereophonic systems, even though many stereo records carry acenter" channel, employ only two loudspeakers.

In the co-pending application of William S. Bachman, Ser. No. 40,510filed May 26, 1970, now abandoned in favor of continuation-in-partapplication Ser. No, 164,675 filed July 21, 1971, and assigned to theassignee of the present invention, there is described a systern forproviding third and fourth playback channels to otherwise two-channelsystems by feeding third and fourth loudspeakers with signalsrespectively representing the sum and difference between the left andright channel signals. The left and right loudspeakers may be located,for example, on opposite sides of a listening area, with theloudspeakers for the two virtual channels positioned at opposite ends ofthe listening area. Each loudspeaker displays the particular informationfed to its channel accompanied by half-power signals from its adjacentchannels. This system provides a pseudo-fourchannel effect, but does notgive complete illusion of each channel appearing independently on itscorresponding loudspeaker.

If a record as described above is played on a monophonic phonograph, thevertically recorded channel will not be reproduced. It is desirable, ofcourse, that such four-channel" records be compatible with the oldermonophonic and stereophonic phonographs, because of the large numbers incurrent use. In other words, it is desirable that when the new medium isplayed on a monophonic or stereophonic phonograph, all channels recordedon the multi-channeled disc be heard with the loudspeaker system of theold phonograph.

SUMMARY OF THE INVENTION A principal object of the present invention isto provide a method and apparatus for reproducing and separatelypresenting on independent loudspeakers four channels of informationrecorded as described above on an otherwise two-track record medium,such as a stereophonic disc record, a two-track tape system havingseparate recording and reproducing heads for each track, or thestereo-multiplex broadcasting system which provides for transmission oftwo independer channels or tracks" of information, such that thelistener experiences the illusion of listening to a corresponding numberof separate sources of sound.

Another object of the invention is to provide a more realistic illusionof four separate channels than is afforded by the system described in.the aforementioned co-pending application.

The invention is applicable to any of the aforementioned presentlyavailable two-track systems of recording and/or transmission on whichtwo of the four separate channels of information are applied in theusual manner, with the other two channels superimposed on the two tracksby applying equal portions of them in phase and relatively shifted inphase, respectively. As applied to a 4545 stereophonic disc record, twoof the channels are recorded on the two separate tracks provided by thewalls of the groove, a third channel is recorded by applying equalportions of the signal in phase to the left" and right channels of thestereophonic cutter to produce lateral modulation of the groove, and thefourth channel is recorded by applying it in equal amounts, butdisplaced in phase, to the left and right" terminals of the cutter toproduce vertical modulation of the record groove. In order that thevertical modulation have a horizontal component to which the oldermonophonic and stereophonic phonographs will be sensitive, the fourthsignal, rather than being split into two equal signals which are appliedout-of-phase, may be applied through a phaseshift network which producestwo signals displaced in phase from each other to cause the cutterstylus to execute an elliptical motion rather than the purely up anddown motion produced by a difference signal.

The information recorded or transmitted on the medium is reproduced byan appropriate transducer to produce two composite signals, a leftsignal which I contains, in addition to the left channel signal, afraction of the third channel and a similar fraction of the fourthchannel, and a right" signal containing the right channel signal, afraction of the third signal, and a similar fraction of the fourthsignal, the latter, however, being in the negative sense. Fourindependent signals, in which the original four channels arepredominant, but each also containing to a lesser degree portions of twoother channels, are derived from the composite signals by appropriatelyadding and subtracting components of the composite signals. An importantaspect of the playback apparatus of the invention is that theinstantaneous amplitudes of the four independent signals delivered tofour corresponding loudspeakers are automatically controlled in responseto the signals then present on the four channels so as to give thelistener a substantially perfect illusion of four separate independentsources of sound.

BRIEF DESCRIPTION OF THE DRAWING An understanding of the foregoing andadditional aspects of this invention may be gained from a considerationof the following detailed description, taken in conjunction with theaccompanying drawings, in which: 1

FIG. 1 is a schematic diagram of a system for recording four channels ofinformation on a stereophonic record;

FIG. 2 is a vector diagram useful in explaining the motion of the cutterstylus in response to application of left, right, center and differencesignals;

FIG. 3 is a cross sectional view ofa fragmentary portion of a recordshowing four record grooves on a greatly enlarged scale, to illustratethe motion of the cutter in response to various signals;

FIG. 4 is a schematic diagram of a prior art stereophonic playbacksystem for providing the illusion of a third channel;

FIG. 5 is a schematic diagram of a system accoring to the invention forrecording four channels on a twotrack stereophonic record;

FIG. 6 is a greatly enlarged illustration of a record grooveillustrating the effect of applying the difference signal to the leftand right channels through a phase shift network;

FIG. 7 is a schematic diagram of one form of playback apparatusembodying the invention;

FIG. 7A is a circuit diagram of a transmission network forming part ofthe system of FIG. 7;

FIG. 8 is a curve showing the transfer characteristic of the logiccircuitry of FIG. 7, useful in explaining the operation of the system;

DESCRIPTION OF THE PREFERRED EMBODIMENTS Although, as noted above, theinvention is applicable to any of a number of known two-track systems,it will be described in the environment of a 45-45 stereophonic discrecord. By way of background, the current method of recordingstereophonic signals including a third or center channel, and a methodof reproducing these signals over a stereophonic two-loudspeaker systemwill be described with reference to FIGS. 1-4. The currently providedleft (L), right (R) and center (C) signals are applied to the twoterminals of a stereophonic cutter 10 having a cutting stylus 12 whichis adapted to cut a groove in the lacquer of a master disc l4, revolvingon a recording turntable (not shown). The C signals is applied through amatrix or signal divider 16 of known configuration resulting inapplication of portions thereof, equivalent to 0.707C, to each of the Land R lines in an additive manner. As is well known in the groovecutting art, the tip of the cutter is capable of motions containedwithin a surface generally perpendicular to the disc in the mannerportrayed by the vector diagram of FIG. 2. When a left signal L isapplied, the stylus executes motions along the arrow L, which is at anangle of 45 to the horizontal, and when an R signal is applied, thestylus motion is along the arrow R, at an angle of 45 to the horizontal.Application of 0.707 parts of C to each of the L and R lines in anadditive manner causes motion of the stylus 1 along the arrow C, equalin magnitude to 0.707 (L+ R), which is of the same magnitude as either Lor R, but directed horizontally. It will be appreciated that instead ofapplying the L, R and C signals directly to the cutter, as shown in FIG.1, they may, in keeping with common practice, be first recorded on atwotrack master tape recorder and the output of the tape reproducerusedto drive the record cutter. Discussion of the difference signal Dil'iififid in FIGS. 1 and 2 will be deferred until later.

The type of groove modulation resulting from the just-describedprocedure is shown in FIG. 3. When only the left signal L is applied,the groove is modulated in accordance with the arrow L, which isessentially confined to one wall of the groove. Similarly, when the Rsignal is applied, the modulation is in the opposite wall of the groovein the direction of the arrow R, which, it will be noted, isperpendicular to the arrow L. Application of equal amounts of the centersignal C to the L and R lines causes both walls of thegroove to besimultaneously and equally modulated in the directions indicated by thearrows L 0.707C and R 0.707C, resulting in horizontal or side to sidetranslation indicated by arrow C.

Apparatus for reproducing a stereophonic record carrying L, R and Csignals recorded in this manner, schematically illustrated in FIG. 4,includes a stereophonic pickup having a cartridge 18 and a stylus 20which enters the groove in the record and is actuated by the groovemodulation to deliver output voltages on the L and R terminals. If onlyL signal modulation is present in the groove, an output signal appearsonly at the L terminal and is amplified by a suitable power amplifier 22and reproduced by a loudspeaker 24. Similarly, when only R signalmodulation is present in the groove, an output voltage appears at onlythe R terminal of the pickup, which is amplified by power amplifier 26and applied to its respective loudspeaker 28. When the groove haslateral modulation consisting of the presence of equal amounts of leftand right signal, then equal signals, namely, 0.707C, appear on both theleft and right loudspeakers, resulting in the appearance of a phantomsource C (shown surrounded by a dashed line circle) midway betweenloudspeakers 24 and 28. However, this illusion is preceptible only tothe centrally located observer 30; when he moves to either side, the Csignal is heard over the nearest loudspeaker unless special precautionsare made to adjust the directional characteristics of the loudspeakerswith respect to the position of the observer.

It will be noted that the described three-channel record is compatiblebecause the L, R and C signals all have a horizontal component and thuswill be heard when played on a monophonic player, which is sensitiveonly to lateral modulation, albeit their relative in- .tensities willnot be in the balance initially intended by the recording director. Inreality, in spite of the introduction of a third channel, theabove'described system reproduces only two independent channels ofinformation. The third channel, C, is contained in both the left andright channels and the listener will, therefore, usually hear itreproduced from the loudspeaker nearest to him. This center channel maybe presented on a separate loudspeaker system, as shown in dotted linesin FIG. 4, and amplifiers are commercially available for this purpose.This permits the observer to perceive the center information withouthaving to locate himself equidistant from the left and right speakers.

Reverting to FIGS. 1-3, a fourth channel, D, may be introduced to thetwo-channel stereophonic system by dividing it into equal parts by amatrix or signal divider 32 and applying them in phase opposition to theleft and right channels. As shown in FIG. 2, application of the D signalin this manner causes motion of the stylus in the vertical direction,along the arrow D, to an extent specified as 0.707 times the amount of Dcontained in the left and right channels subtracted from each other;i.e., 0.707 (L 4 R). As seen in FIG. 3, this causes the left and rightmotions of the stylus to be out-of-phase relative to each other,resulting in up and down motion. When vertical modulation is reproducedby the system of FIG. 4, the loudspeaker cones are driven in oppositedirections, resulting in out-of-phase sound pressures applied to theears of the listener, and since this condition of pressure on the earsdoes not correspond to any known normal listening condition, theobserver is unable to localize the sound. The difference signal Dappears at some indefinite point in space, shown as D in a dashedcircle, and the listener is unable to locate its whereabouts.Furthermore, some listeners of such outof-phase sound have complained ofa peculiar pressure in the ears sensation. This is in part overcome,however, by the system described in the aforementioned Bachmanapplication Ser. No. 164,675 wherein the difference signal, as well asthe center" signal, are reproduced on separate loudspeakers.

To afford better compatibility with monophonic and conventionalstereophonic players, while at the same time improving the illusion offour separate channels during playback, the difference signal D ispreferably applied in the manner suggested in applicant Bauers articleentitled Some Techniques Toward Better Stereophonic Perspective, IEEETRANSACTIONS ON AUDIO, Vol. AU-l 1, No. 3, May-June, 1963. In keepingtherewith, and as is illustrated in FIG. 5, instead of applying thedifference signal equally and oppositely to the left and right channelsas in the circuit of FIG. 1, the D signal is applied through anacoustical phase shift network 32 which splits the incoming signal intotwo equal amplitude signals D and D each containing all of thefrequencies of the D signal, but displaced in phase with respect to eachother. Relative phase displacements in the range of 1 to 170 have beensuccessfully used, with an angle of 135 being particularly suitable. Itcan be readily demonstrated that when the two signals are thus displacedrelative to each other, the tip of the stylus instead of undergoing apurely up and down motion as shown in FIG. 3, executes the ellipticalmotion illustrated in FIG. 6. The limits of stylus motion are shown bythe dashed lines and the direction of motion of the ellipse depends onwhether D leads D or vice versa. The important consideration is that thegrove has a horizontal component defined by the horizontal width of theellipse, whereby both monophonic and stereophonic phonographs willreproduce all four signals; that is, the record with four separatechannels will be fully compatible with the older playback systems,albeit with monophonic systems the signal D is attenuated by about 8db.

Although the description thus far has been concerned primarily withrecording four separate channels of information on a two-trackstereophonic record, it will be recognized by ones skilled in the artthat similar techniques may be employed to record similar signals on atwo-track tape system, for example, or to transmit comparable signalsover the known stereo-multiplex system of broadcasting. The descriptionto follow will be directed to apparatus for reproducing the signals in amanner such that the four channels may be separately and independentlypresented on four different loudspeakers. Except for the transducermeans required to derive the. signals from the record medium, theplayback apparatus of the invention is applicable to all of such knowntwo-track systems.

Referringnow to FIG. 7, and considering the playback apparatus asapplied to a disc recorded as described above, the recorded signals arederived by a stereophonic pickup of the type illustrated in FIG. 4, andmay be applied to the system directly or through a pair of suitableamplifiers 40 and 42. It will be evident from what has been said earlierthat the left signal, labeled L is a composite signal including inaddition to the L signal, 0.70 of the signal C and 0.707 of the signalD. Similarly, the right signal, R contains the right channel signal R,0.707 of the signal C, and 0.707 of the signal D, the latter beingout-of-phase in the two cases, It is evident that if the outputs ofamplifiers 40 and 42 were connected to two respective loudspeakers, thereproduction would be equivalent to that of a conventional two-channelstereophonic system. In the present system, however, the L and R signalsare also applied to a pair of signal dividing circuits 44 and 46 ofknown configuration which, by appropriate addition and subtraction ofcomponents of the composite signals, produce four signals L, C, D and R,respectively containing the four predominant channels L, C, D and R,with each also containing to a lesser degree portions of two otherchannels. Therefore, the signals delivered to the respectiveloudspeakers L, C, R and D, after amplification by suitable poweramplifiers 48, 50, S2 and 54, respectively, are not composed of the pureinformation of the corresponding L, C, D and R channels, but rather arediluted with portions of the information from the adjacent channels.

In accordance with the invention, the instantaneous amplitudes of thesignals delivered to the four loudspeakers are controlled by logiccircuitry contained within the dashed line enclosure 56 in such a mannerthat a listener is given a substantially perfect illusion of fourseparate independent sources of sound. This ob jective is achieved byreason of the character of the music normally reproduced on a phonographrecord, aided by a phenomenon known in acoustical science as theprecedence effect. In most musical selections, the individual performersdo not play continuously, but rather, produce a constantly varyingpattern of attacks,

decays, percussion sounds, etc., which do not occur simultaneously butare interleaved with each other. For example, first the sound of a drummay appear on channel L, followed by the sound of a cymbal on channel R,followed by the voice of a soloist pronouncing various syllables onchannel C, etc. If one is able to switch the loudspeaker system in sucha way that a loudspeaker is turned on, or the signal preferentiallyamplified, each time a particular impulsive or percussive sound isstarted in its channel, while the remaining loudspeakers arecorrespondingly turned off or attenuated, the listener will have fixedthis attention upon the particular sound coming from that loudspeaker,and even if the sound is transferred to another loudspeaker, he willhave an illusion of the sound proceeding from the first loudspeaker.This results from the well known precedence effect which is based on theobservation that when a sound originates from a given loudspeaker, andthen gradually is switched into another loudspeaker, the listenercontinues to hear the sound coming from the given loudspeaker long afterit has ceased to be the true source.

Accordingly, the function to be performed by the control logic portion56 of the playback apparatus is to identify which channel has thestrongest signal at any instant in time and to turn that channel on, orto preferentially increase its gain, while attenuating or turning offthe remaining channels. As the sound diminishes in the channel firstidentified and another sound appears on a different channel, the logiccircuit rapidly attenuates the gain in the first channel and in creasesthe gain in a different channel. It is useful to think of the action ofthis logic in terms of the following truth table:

If there are sound signals in the system it logically follows that:

If L signal 0, only R signal is present If R signal 0, only L signal ispresent If C signal 0, only D signal is present If D signal 0, only Csignal is present.

It will be observed that this is a form of negative logic; i.e., thelack of sound in channel L, for'example, or the diminution thereof,serves to turn on or to enhance the gain of the R channel, etc., inaccordance with the truth table.

Turning now to a discussion of the logic system, the outputs fromamplifiers 40 and 42 (if provided) are respectively applied to gaincontrol amplifiers 58 and 60, the gains of which are controlled inunison as indicated by the connection 62 therebetween.

For reasons which will be better understood after considering the logiccircuit, it is desirable to apply the L and R signals to amplifiers 58and 60 through respective signal modifying networks 59 and 61. These twonetworks are identical and exhibit transmission characteristics whichresemble the equal loudness contour of the human ear at moderateloudness level and over the audio range of interest. A set of equalloudness contours are illustrated and described in an article byapplicant Bauer and Emil Torick entitled Researches in LoudnessMeasurement," IEEE TRANSACTIONS ON AUDIQ AND ELECTROACOUSTICS, Vol.AU-14, No. 3, pp. l4l-l5l, 1966. The 70 phon equal loudness contourdeveloped in the study described in this article is shown in blocks 59and 61 in the inverted, or sensitivity, form. It will be noted thatthere is a peak in the transmission characteristic at the higherfrequencies (at approximately 4 KI-Iz), which may be of theorder of 8db, is essentially constant from approximately 2 KHz down to about 50Hzat which it exhibits a drop of approximately 5 db. The function of thenetworks 59 and 61 is to so shape the signals delivered by thetransducer to the gain control amplifiers 58 and 60 so that the signalswitching logic (the operation of which is about to be described) willplace the respective L, C, D and R signals in their proper channels onthe basis of their relative loudness, rather than their energy content.For example, the weighting curves of networks 59 and 61 would precludethe low frequency, but high energy, signal produced by a drum fromincorrectly switching the higher frequency, lower energy, signalproduced by a piccolo, for example.

While the transmission characteristic exhibited by circuits 59 and 61may be obtained in a number of ways, a preferred embodiment is shown inFIG. 7A, consisting of a high resistance R1, in parallel with a seriesbranch containing a lower valued resistor R2, a capacitor Cl and aninductor L1, followed by a small resistor R3 to ground and a seriesresistor to amplifier 58 (or amplifier 60). The values of the componentsin the series branch are selected so that in cooperation with theparallel resistor R1, the circuit produces the peak in the transmissioncharacteristic centered at about 4 KHz. Typical values of the componentsused in the circuit of FIG. 7A are:

R1 5.1 Kohms R2 1.6 Kohms R3 91 ohms R4 1.0 Kohms C1 0.01 ,uf. L1 300mh.

It will be understood that the exact shape of the characteristic may bemodified by experiment to provide the best results with any particulardecoder, or it may be desirable to provide an adjustment to permit theuser to adjust the decoder for any particular type of music, or anyparticular type of listening environment.

Returning now to the control circuit itself, the outputs of amplifiers58 and 60 are separated by separating circuits 64 and 66 into fourseparate outputs L, C, D" and R in the same manner as the outputs L, C,Dand R are produced by circuits 44 and 46. These two sets of fouroutputs thus resemble each other in musical content, but the former setis held at a uniform level, despite variations in the dynamic range ofthe record (as modified by circuits 59 and 61), by the action of gaincontrol amplifiers 58 and 60. To achieve this constant output level, theL, C, D and R signals are rectified by rectifiers 68, 70, 72 and 74,respectively, and summed by isolating resistors 76, 78, and 82 todevelop a sum signal across a common resistor 84. The voltage developedacross resistor 84 is applied over conductor 86 to the gain control lead62 of amplifiers 58 and 60, which are operative in response thereto tokeep the average voltage across resistor 84 substantially constant. Thegain control action is enhanced by connecting four capacitors 88, 90, 92and 94 across resistors 76, 78, 80 and 82, respectively, whereby therectified voltage represents the envelope of the wave rather than itsinstantaneous value. The automatic gain control action maintains the sumof the voltages across the resistors 76, 78, 80 and 82 constant becausethe voltage across the relatively smaller resistor 84 is the sum of thefour rectified voltages.

The action of the gain control circuit can best be un derstood byconsideration of several illustrative examples. Let it be assumed thatthe system is playing a record which contains a single signal, say aleft signal L, the amplitude of which is arbitrarily asigned a value ofunity. It follows from the vector diagram of FIG. 2 that the voltagese,, e 6,, and e, respectively developed across resistor 76, 78, 80 and82 would then have relative values of 1, 0.707, 0.707, and 0. The sum ofthese voltages is 2.414, and the gain control amplifiers 58 and 60 aredesigned to maintain the voltage across resistor 84 at this valueregardless of the sound level in channel L. This represents onecondition of circuit operation and will be considered later in furtherdetail.

Now let it be assumed that only the C signal is present, in which casethe voltages e,, c e and 2, will be 0.707, 1,0 and 0.707, respectively.It will be noted that the sum of these voltages is also 2.414.

Now, if incoherent signals are simultaneously present on channels L andC (say, signals resembling white noise; that is, signals emanating fromtwo different sources), the voltages corresponding to the sum of thesechannels will be equal to the square root of the sum of the squares ofboth voltages. Therefore, the

unadjusted sum of the signals L and C will be as shown in the followingTable I, namely, e 1.223, e =l.223, e,;=0.707 and e,=0.707. The sum ofthese four voltages being 3.860, the automatic gain control amplifiersinstantaneously adjust the component voltages so as to total 2.414.Thus, each of these voltages is proportionally reduced by the fraction2414/3860 to give adjusted values of e,=0.760, e ==0.760, e =0.440 ande,=0.440.

Consider now the presence ofa third channel, say, R. The voltagescorresponding to this channel acting alone are e,=0, e =0.707, e =0.707and e,=l. Again, assuming that all three signal are white-noise-likeincoherent signals, the resulting unadjusted values of e,, e,, e and efor the (L-l-C-l-R) condition are obtained by taking the square root ofthe sum of the squares of the individual voltages for these threechannels, and are shown in Table I as being 1.223, 1.414, 1.223 and1.223, respectively. These four voltages add up to a total of 5.083,whereby the automatic gain control amplifiers cause these componentvoltages to immediately reduce by the factor 2.414/5083, resulting inthe adjusted voltages shown in Table I.

If a fourth voltage corresponding to the D channel is now added, itturns out that each of the four summed voltages has a value of 1.414,which, when acted upon by the automatic gain control amplifiers arereduced to a value to total 2.414, with the result that each of thecomponent voltages has a value of 0.6.

Returning for a moment to the right-hand portion of Fig. 7, the foursignals L, C, D' and R are applied to respective gain control amplifiers96, 98, and 102, and then to four loudspeakers L, C, D, and R,respectively, with intermediate amplification, if necessary, provided byamplifiers 48, 50, 52 and 54. The gaincontrol amplifiers are keyelements of the invention in that they control the gain of the signalsapplied to the respective loudspeakers in accordance with the logicdescribed earlier.

If the aforementioned signals are incoherent, but exhibit definitefrequency-like character, then the added peak value in the logic willapproximate the sum of the signals, rather than the root-mean-squarevalue. This, however, does not alter the analysis made hereinabove.

The requisite control is accomplished by applying the component voltagese e e and e, tothe gain control amplifiers as follows: e, is applied toamplifier 102 to thereby dcontrol the R signal; e, is applied toamplifier 96 to control the L signal; 2 is applied to amplifier 100 tocontrol the D signal; and e,, is applied to amplifier 98 to control theC signal. For convenience in implementing the above-discussed truthtable, the rectifiers 68, 70, 72 and 74 are connected in a negativelyconducting fashion so that voltages e,, 2,, e and e, and the variouscombinations thereof shown in Table I are negative voltages.Additionally, the control circuits of gain control amplifiers 96, 98,100 and 102 are positively biased with a relative voltage of 0.6 voltsby means of batteries 104, 106, 108 and 110, respectively.

The gain control amplifiers have thecontrol characteristic shown in FIG.8, which indicates that when a voltage of +0.6 volt is applied to thegain control termi' nal of the amplifier, its gain is maximum, at avalue designated as unity. When the applied voltage is reduced to zero,the gain of the amplifier is decreased to 0.707 of maximum; that is, thegain is down 3 db. The charac teristic then falls rapidly such that whenthe gain control voltage is 0.67 volts, the gain is reduced to zero andthe amplifier is turned off.While the appropriate control characteristicmay be obtained in a number of ways, it is conveniently obtained byusing an integrated DC amplifier Model CA3000, available from RCA, inthe circuit configuration described on page 6 of RCA Integrated CircuitsApplication Note ICAN-5030 printed in Sept. 1967 and then available tothe industry.

The action of the logic circuit 56 for the four conditions of L signalonly, two signals, such as L C, three signals, such as L C R incombination, and four random incoherent signals L C R D, will now beexamined. It will be observed from Table I that when only the L signalis present, e =0 whereby the +0.6 volt bisa turns amplifier 96 (whichcontrols the L signal) fully on", whereas negative voltages of 1.0,0.707 and 0.707 are respectively applied to the amplifiers controllingthe R, D and C signals. Consequently, amplifiers 102, 100 and 98 arebiased beyond cutoff so as to have zero gain, and the L signal, whichotherwise would also appear in loudspeakers C and D, appears, only inloudspeaker L. By similar analysis it may be shown that any one signalappearing in any one individual channel will turn on only the gaincontrol amplifier appropriate to that signal.

Considering now the condition when L and C signals are both prsent, itwill be seen from Table l and from the notations in FIG. 8 that the O.76volt applied to amplifiers 100 and 102 exceed the +0.6 volt bias therebycuasing loudspeakers D and R to be turned off;" however, the O.44O voltapplied to amplifiers 96 and 98 cuases them both to be turned on, withonly a slight reduction from unity gain, whereby the L and C signalsappear in their respective loudspeakers L and C.

Similarly, when L, C and R signals are simultaneously present, it willbe seen from Table I and the notations on FIG. 8 that 2 is sufficientlynegative to cause amplifier 100 (which controls the signal applied toloudspeaker D) to be turned off whereas the 0.577 volts applied to theother three gain control amplifiers causes them to remain on, but withtheir gains reduced by approximately 2.5 db, so that the L, C and Dsignals are reproduced on their respective loudspeakers.

Finally, when L, C, R and D signals are all present in equal amounts,the respective gain control voltages e,, e e and e, are all 0.6 voltswhich allows all four amplifiers 96, 98, 100 and 102 to be turned on andto apply the signals to their respective loudspeakers. However, it willbe seen from the control characteristic that the gain of the amplifiersis reduced by approximately 3 db when all four signals are present.Thus, the total sound energy reproduced by the loudspeakers remainsessentially constant regardless of the number of signals present.

It is evident from the foregoing that the logic system is operative toturn on those loudspeakers which correspond to the predominant soundsinstantaneously present in the system, thereby accomplishing animportant object of the invention. In actual practice, all signalsseldom occur simultaneously, but rather there is a constant interplay ofthe various instruments which turns the loudspeakers on and offin amanner to give a completely natural illusion of four separate sources ofsound being present and reproduced over the four loudspeakers.

For best operation, it is preferable that the time constants of therectifier circuits 68-74 have a very rapid attack time of the order of0.1 milliseconds and relatively slow decay time of approximately 10milliseconds. Likewise, the attack time of the gain control amplifiers96-102 should be extremely rapid-of the order of 0.1 millisecondwhile adecay time of the order of 0.4 second has been found satisfactory. It isto be understood, however, that these attack and decay times may beadjusted between relatively wide limits without seriously impairing theperformance of the circuit.

We claim:

1. Apparatus for transforming a multi-channel program including at leastfirst, second and third separate audio program signals into twocomposite signals, said apparatus comprising,

at least first, second and third input terminals to which said separatefirst, second and third program signals are respectively applied,

first and second output terminals,

means connected between said first and second input terminals and saidfirst and second output terminals, respectively, operative to transfersubstantially equal amplitude proportions of said first and secondprogram signals without relative phase change to said first and secondoutput terminals, respectively, and

means connected between said third input terminal and both said firstand second output terminals operative to transfer substantially equalreduced amplitude proportions of said third program signal to both saidfirst and second output terminals and including phase-shifting meansoperative to provide a differential phaseshift angle between theproportion of said third program signal transferred to said firstterminal and the proportion of said third program signal transferred tosaid second output terminal, which phase-shift angle has a value in therange between 1 10 and and remains substantially constant over the rangeof frequencies of said third program signal.

2. Apparatus in accordance with claim 1 further including a fourth inputterminal to which a fourth program signal is applied, and

means connected between said fourth input terminal and both said firstand second output terminals operative to transfer substantially equalreduced amplitude in-phase proportions of said fourth program signal toboth said first and second output termi-

1. Apparatus for transforming a multi-channel program including at leastfirst, second and third separate audio program signals into twocomposite signals, said apparatus comprising, at least first, second andthird input terminals to which said separate first, second and thirdprogram signals are respectively applied, first and second outputterminals, means connected between said first and second input terminalsand said first and second output terminals, respectively, operative totransfer substantially equal amplitude proportions of said first andsecond program signals without relative phase change to said first andsecond output terminals, respectively, and means connected between saidthird input terminal and both said first and second output terminalsoperative to transfer substantially equal reduced amplitude proportionsof said third program signal to both said first and second outputterminals and including phase-shifting means operative to provide adifferential phase-shift angle between the proportion of said thirdprogram signal transferred to said first terminal and the proportion ofsaid third program signal transferred to said second output terminal,which phase-shift angle has a value in the range between 110* and 170*and remains substantially constant over the range of frequencies of saidthird program signal.
 2. Apparatus in accordance with claim 1 furtherincluding a fourth input terminal to which a fourth program signal isapplied, and means connected between said fourth input terminal and bothsaid first and second output terminals operative to transfersubstantially equal reduced amplitude in-phase proportions of saidfourth program signal to both said first and second output terminals.